These results obtained by Gorup-Besanez are possibly not altogether above criticism, for later workers showed that copper was absorbed to some extent by plants grown in water cultures, and if that is so it seems unlikely that no absorption should take place from soil. Nevertheless, the absorption is very slight, for apparently living protoplasm is very resistant to copper osmotically. Otto showed that beans, maize and peas can have their roots for a long time in a relatively concentrated solution of copper sulphate, and yet take up very little copper indeed, but analyses do reveal slight traces after a sufficient interval of time of contact has elapsed. Berlese and Sostegni indicate that the roots of plants grown in water culture in the presence of bicarbonate of copper showed traces of copper. Verschaffelt (1905) devised an ingenious method of estimating the toxic limits of plant poisons, though it is rather difficult to see how the method can be put to practical use with water culture and soil experiments. Living tissues increase in weight when put into water on account of the absorption of water. Dead tissues do not, as they have lost their semi-permeable characteristics, so a decrease in weight takes place owing to part of the water passing out. This principle is applied by Verschaffelt to determine the “mortal limit” of external agents in their action on plant tissues. Root of beetroot, potato tuber, aloe leaves, and parts of other plants rich in sugar all came under review. The parts were cut into small pieces weighing about 3–5 grams, dried with filter paper, weighed, and plunged into solutions of copper sulphate of varying strengths from ·001–·004 gm. mol. per litre, and left for 24 hours. After drying and again weighing all were heavier owing to the absorption of water. The pieces were then immersed in pure water for another period of 24 hours, when after drying and weighing, those from the weaker strengths of copper sulphate (·001–·002) had absorbed yet more water, while those from higher concentrations (·003–·004) had lost weight. So the author assumes that for such pieces of potato the limit of toxicity lies between ·002 and ·003 gm. mol. copper sulphate per litre.